The present invention relates generally to the detection of abnormal or cancerous tissue, and more particularly, to the detection of changes in the electrophysiological characteristics of abnormal or cancerous tissue and to changes in those electrophysiological characteristics related to the functional, structural and topographic (the interaction of shape, position and function) relationships of the tissue during the development of malignancy. These measurements are made in the absence and presence of pharmacological and hormonal agents to reveal and accentuate the electrophysiological characteristics of abnormal or cancerous tissue.
Cancer is a leading cause of death in both men and women in the United States. Difficulty in detecting abnormal pre-cancerous or cancerous tissue before treatment options become non-viable is one of the reasons for the high mortality rate. Detecting of the presence of abnormal or cancerous tissues is difficult, in part, because such tissues are largely located deep within the body, thus requiring expensive, complex, invasive, and/or uncomfortable procedures. For this reason, the use of detection procedures is often restricted until a patient is experiencing symptoms related to the abnormal tissue. Many forms of cancers or tumors, however, require extended periods of time to attain a detectable size (and thus to produce significant symptoms or signs in the patient). It is often too late for effective treatment by the time the detection is performed with currently available diagnostic modalities.
Breast cancer is the most common malignancy affecting women in the Western World. The reduction in mortality for this common disease depends on early detection. The mainstay of early detection are X-ray mammography and clinical breast examination. Both are fraught with problems of inaccuracy. For example, mammography has a lower sensitivity in women with dense breasts, and is unable to discriminate between morphologically similar benign or malignant breast lesions.
Clinical breast examinations are limited because lesions less than one cm are usually undetectable and larger lesions may be obscured by diffuse modularity, fibrocystic change, or may be too deep in the breast to enable clinical detection. Patients with positive mammogaphic or equivocal clinical findings often require biopsy to make a definitive diagnosis. Moreover, biopsies may be negative for malignancy in up to 80% of patients.
Accordingly, mammography and clinical breast examination have relatively poor specificity in diagnosing breast cancer. Therefore many positive mammographic findings or lesions detected on clinical breast examination ultimately prove to be false positives resulting in physical and emotional trauma for patients. Improved methods and technologies to identify patients who need to undergo biopsy would reduce healthcare costs and avoid unnecessary diagnostic biopsies.
Other technologies have been introduced in an attempt to improve on the diagnostic accuracy attainable with mammography and clinical breast examination alone. Breast ultrasound is helpful in distinguishing between cystic or solid breast lesions and may be useful in guiding needle or open biopsies. However, such techniques are unable to determine whether a solid mass, or calcifications are benign or malignant. Magnetic resonance imaging has been introduced in an attempt to improve on the accuracy of mammography. Its high cost and low specificity limit its general applicability for diagnosing and screening for breast cancer. Nuclear imaging with Positron Emission Tomogaphy (PET) has a lower sensitivity for small lesions, but is limited by cost.
One proposed method for early detection of cancerous and pre-cancerous tissue includes measuring of the electrical impedance of biological tissue. For example, U.S. Pat. No. 3,949,736 discloses a low-level electric current passed through tissue, with a measurement of the voltage drop across the tissue providing an indirect indication of the overall tissue impedance. This method teaches that a change in impedance of the tissue is associated with an abnormal condition of the cells composing the tissue, indicating a tumor, carcinoma, or other abnormal biological condition. This disclosure, however, does not discuss either an increase or decrease in impedance associated with abnormal cells, nor does it specifically address tumor cells.
The disadvantage of this and similar systems is that the DC electrical properties of the epithelium are not considered. Most common malignancies develop in an epithelium (the cell layer that lines a hollow organ, such as the bowel, or ductal structures such as the breast or prostate), that maintains a transepithelial electropotential. Early in the malignant process the epithelium loses its transepithelial potential, particularly when compared to epithelium some distance away from the developing malignancy. The combination of transepithelial electropotential measurements with impedance are more accurate in diagnosing pre-cancerous and cancerous conditions.
Another disadvantage of the above referenced system is that the frequency range is not defined. Certain information is obtained about cells according to the range of frequencies selected. Different frequency bands may be associated with different structural or functional aspects of the tissue. See, for example, F. A. Duck, Physical Properties of Tissues, London: Academic Press, 2001; K. R. Foster, H. P. Schwan, Dielectric properties of tissues and biological materials: a critical review, Crit. Rev. Biomed. Eng., 1989, 17(1): 25-104. For example at high frequencies (>1 GHz) molecular structure has a dominating effect on the relaxation characteristics of the impedance profile. Relaxation characteristics include the delay in the response of a tissue to a change in the applied electric field. For example, an applied AC current results in voltage change across the tissue which will be delayed or phase shifted, because of the impedance characteristics of the tissue. Relaxation and dispersion characteristics of the tissue vary according to the frequency of the applied signal.
At lower frequencies, such as <100 Hz, or the so called α-dispersion range, alterations in ion transport and charge accumulations at large cell membrane interfaces dominate the relaxation characteristics of the impedance profile. In the frequency range between a few kHz and 1 MHz, or the so-called β-dispersion range, cell structure dominates the relaxation characteristics of the epithelial impedance profile. Within this range at low kHz frequencies, most of the applied current passes between the cells through the paracellular pathway and tight junctions. At higher frequencies in the β-dispersion range the current can penetrate the cell membrane and therefore passes both between and through the cells, and the current density will depend on the composition and volume of the cytoplasm and cell nucleus. Characteristic alterations occur in the ion transport of an epithelium during the process of malignant transformation affecting the impedance characteristics of the epithelium measured at frequencies in the α-dispersion range. Later in the malignant process, structural alterations with opening of the tight junctions and decreasing resistance of the paracellular pathways, together with changes in the composition and volume of the cell cytoplasm and nucleus, affect the impedance measured in the β-dispersion range.
Another disadvantage with the above referenced system is that the topography of altered impedance is not examined. By spacing the measuring electrodes differently the epithelium can be probed to different depths. The depth that is measured by two surface electrodes is approximately half the distance between the electrodes. Therefore electrodes 1mm apart will measure the impedance of the underlying epithelium to a depth of approximately 500 microns. It is known, for example, that the thickness of bowel epithelium increases at the edge of a developing tumor to 1356±208μ compared with 716±112μ in normal bowel. D. Kristt, et al. Patterns of proliferative changes in crypts bordering colonic tumors: zonal histology and cell cycle marker expression. Pathol. Oncol. Res 1999; 5(4): 297-303. Thickening of the ductal epithelium of the breast is also observed as ductal carcinoma in-situ develops. By comparing the measured impedance between electrodes spaced approximately 2.8 mm apart and compared with the impedance of electrodes spaced approximately 1.4 mm apart, information about the deeper and thickened epithelium may be obtained. See, for example, L. Emtestam & S. Ollmar. Electrical impedance index in human skin: measurements after occlusion, in 5 anatomical regions and in mild irritant contact dermatitis. Contact Dermatitis 1993; 28(2): 104-108.
Another disadvantage of the above referenced methods is that they do not probe the specific conductive pathways that are altered during the malignant process. For example, potassium conductance is reduced in the surface epithelium of the colon early in the malignant process. By using electrodes spaced less than 1 mm apart with varying concentrations of potassium chloride the potassium conductance and permeability may be estimated in the surface epithelium at a depth from <500μ to the surface.
A number of non-invasive impedance imaging techniques have been developed in an attempt to diagnose breast cancer. Electrical impedance tomography (EIT) is an impedance imaging technique that employs a large number of electrodes placed on the body surface. The impedance measurements obtained at each electrode are then processed by a computer to generate a 2D or 3D reconstructed tomographic image of the impedance and its distribution in 2 or 3 dimensions. This approach relies on the differences in conductivity and impedivity7 between different tissue types and relies on data acquisition and image reconstruction algorithms which are difficult to apply clinically.
The majority of EIT systems employ “current-driving mode,” which applies a constant AC current between two or more current-passing electrodes, and measures the voltage drop between other voltage-sensing electrodes on the body surface. Another approach is to use a “voltage-driving approach,” which applies a constant AC voltage between two or more current-passing electrodes, and then measures the current at other current-sensing electrodes. Different systems vary in the electrode configuration, current or voltage excitation mode, the excitation signal pattern, and AC frequency range employed.
Another disadvantage with using EIT to diagnose breast cancer is the inhomogeneity of breast tissue. The image reconstruction assumes that current passes homogeneously through the breast tissue which is unlikely given the varying electrical properties of different types of tissue comprising the breast. In addition image reconstruction depends upon the calculation of the voltage distribution on the surface of the breast from a known impedance distribution (the so called forward problem), and then estimating the impedance distribution within the breast from the measured voltage distribution measured with surface electrodes (the inverse problem). Reconstruction algorithms are frequently based on finite element modeling using Poisson's equation and with assumptions with regard to quasistatic conditions, because of the low frequencies used in most EIT systems.
Other patents, such as U.S. Pat. Nos. 4,955,383 and 5,099,844, disclose that surface electropotential measurements may be used to diagnose cancer. Empirical measurements, however, are difficult to interpret and use in diagnosis. For example, the above referenced inventions diagnose cancer by measuring voltage differences (differentials) between one region of the breast and another and then comparing them with measurements in the opposite breast. Changes in the measured surface potential may be related to differences in the impedance characteristics of the overlying skin. This fact is ignored by the above referenced and similar inventions, resulting in a diagnostic accuracy of 72% or less. J. Cuzick et al. Electropotential measurements as a new diagnostic modality for breast cancer. Lancet 1998; 352(9125): 359-363; M. Faupel et al. Electropotential evaluation as a new technique for diagnosing breast lesions. Eur. J. Radiol. 1997; 24 (1): 33-38. Neither AC impedance, or surface DC measurement approaches, measure the transepithelial breast DC potential or AC impedance characteristics of the breast epithelium.
Other inventions that use AC measurement, such as U.S. Pat. No. 6,308,097, also have a lower accuracy than may be possible with a combination of DC potential measurements and AC impedance measurements, that also measure the transepithelial electrical properties of mammary epithelium. Electrical impedance scanning (EIS) also known as electrical impedance mapping (EIM) avoids the limitations of complex image reconstruction encountered with EIT. The above referenced system diagnoses cancer by only measuring decreased impedance (increased conductance) and changes in capacitance over a cancer. It does not measure the mammary transepithelial impedance characteristics of the breast. There are several other limitations to this approach. Inaccuracies may occur because of air bubbles. Underlying bones, costal cartilages, muscle and skin may result in high conductance regions, which produce false positives. Depth of measurement is limited to 3-3.5 cm, which will result in false negatives for lesions on the chest wall. It is also not possible to localize lesions using this approach.
Another potential source of information for the detection of abnormal tissue is the measurement of transport alterations in the epithelium. Epithelial cells line the surfaces of the body and act as a barrier to isolate the body from the outside world. Not only do epithelial cells serve to insulate the body, but they also modify the body's environment by transporting salts, nutrients, and water across the cell barrier while maintaining their own cytoplasmic environment within fairly narrow limits. One mechanism by which the epithelial layer withstands the constant battering is by continuous proliferation and replacement of the barrier. This continued cell proliferation may partly explain why more than 80% of cancers are of epithelial cell origin. Moreover, given their special abilities to vectorially transport solutes from blood to outside and vice versa, it appears that a disease process involving altered growth regulation may have associated changes in transport properties of epithelia.
It is known that the addition of serum to quiescent fibroblasts results in rapid cell membrane depolarization. Cell membrane depolarization is an early event associated with cell division. Depolarization induced by growth factors appears biphasic in some instances but cell division may be stimulated without depolarization. Cell membrane depolarization is temporally associated with Na+ influx, and the influx persists after repolarization has occurred. Although the initial Na+ influx may result in depolarization, the increase in sodium transport does not cease once the cell membrane has been repolarized, possibly due to Na/K ATPase pump activation. Other studies also support the notion that Na+ transport is altered during cell activation. In addition to altered Na+-transport, K+-, and Cl−-transport is altered during cell proliferation.
A number of studies have demonstrated that proliferating cells are relatively depolarized when compared to those that are quiescent or non-dividing. Differentiation is associated with the expression of specific ion channels. Additional studies indicate that cell membrane depolarization occurs because of alterations in ionic fluxes, intracellular ionic composition and transport mechanisms that are associated with cell proliferation.
Intracellular Ca2+ (Ca2+i) and pH (pHi) are increased by mitogen activation. Cell proliferation may be initiated following the activation of phosphatidylinositol which releases two second messengers, 1,2-diacylglycerol and inosotol-1,4,5-triphosphate, which triggers Ca2+i release from internal stores. Ca2+i and pHi may then alter the gating of various ion channels in the cell membrane, which are responsible for maintaining the voltage of the cell membrane. Therefore, there is the potential for interaction between other intracellular messengers, ion transport mechanisms, and cell membrane potential. Most studies have been performed in transformed and cultured cells and not in intact epithelia during the development of cancer.
It was known for some time that cancer cells are relatively depolarized compared with non-transformed cells. It has been suggested that sustained cell membrane depolarization results in continuous cellular proliferation, and that malignant transformation results as a consequence of sustained depolarization and a failure of the cell to repolarize after cell division. C. D. Cone Jr., Unified theory on the basic mechanism of normal mitotic control and oncogenesis. J. Theor. Biol. 1971; 30(1): 151-181; C. D. Cone Jr., C. M. Cone. Induction of mitosis in mature neurons in central nervous system by sustained depolarization. Science 1976; 192(4235): 155-158; C. D. Cone, Jr. The role of the surface electrical transmembrane potential in normal and malignant mitogenesis. Ann. N.Y. Acad. Sci. 1974; 238: 420-435. A number of studies have demonstrated that cell membrane depolarization occurs during transformation and carcinogenesis. Other studies have demonstrated that a single ras-mutation may result in altered ion transport and cell membrane depolarization. Y. Huang, S. G. Rane, Single channel study of a Ca(2+)-activated K+ current associated with ras induced cell transformation. J. Physiol. 1993; 461: 601-618. For example, there is a progressive depolarization of the colonocyte cell membrane during 1,2 dimethylhydrazine (DMH)-induced colon cancer in CF1 mice. The VA (apical membrane voltage) measured with intracellular microelectrodes in histologically “normal” colonic epithelium depolarized from −74.9 mV to −61.4 mV after 6 weeks of DMH treatment and to −34 mV by 20 weeks of treatment. The cell membrane potential in a benign human breast epithelial cell line (MCF-10A) was observed to be −50±4 mV (mean±SEM) and was significantly depolarized at −35±1 mV (p<0.002) in the same cell line after ras-transformation (the MCF-10AT cell line).
While epithelial cells normally maintain their intracellular sodium concentration within a narrow range, electronmicroprobe analysis suggests that cancer cells exhibit cytoplasmic sodium/potassium ratios that are three to five times greater than those found in their non-transformed counterparts. These observations partly explain the electrical depolarization observed in malignant or pre-malignant tissues, because of the loss of K+ or Na+ gradients across the cell membrane.
In addition to cell membrane depolarization, and altered intracellular ionic activity, other studies have shown that there may be a decrease in electrogenic sodium transport and activation of non-electrogenic transporters during the development of epithelial malignancies. These changes may affect or occur as a consequence of altered intracellular ionic composition.
In addition to cell membrane depolarization, and altered intracellular ionic activity, other studies have shown that there may be a decrease in electrogenic sodium transport and activation of non-electrogenic transporters during the development of epithelial malignancies. These changes may occur as a consequence of altered intracellular ionic composition. Other specific ion transport alterations have been described in colon, prostate, breast, uterine cervix, melanoma, urothelium, and pancreas during proliferation, differentiation, apoptosis, and carcinogenesis.
Apoptosis or physiological cell death is down-regulated during the development of malignancy. Ion transport mechanisms affected by apoptosis include the influx of Ca2+, non-selective Ca2+-permeable cation channels, calcium-activated chloride channels, and K+-Cl31 -cotransport. J. A. Kim et al. Involvement of Ca2+ influx in the mechanism of tamoxifen-induced apoptosis in Hep2G human hepatoblastoma cells. Cancer Lett. 1999; 147(1-2): 115-123; A. A. Gutierrez et al. Activation of a Ca2+-permeable cation channel by two different inducers of apoptosis in a human prostatic cancer cell line. J. Physiol. 1999; 517 (Pt. 1): 95-107; J. V. Tapia-Vieyra, J. Mas-Oliva. Apoptosis and cell death channels in prostate cancer. Arch. Med. Res. 2001; 32(3): 175-185; R. C. Elble, B. U. Pauli. Tumor Suprression by a Proapoptotic Calcium-Activated Chloride Channel in Mammary Epithelium. J. Biol. Chem. 2001; 276(44): 40510-40517.
Loss of cell-to-cell communication occurs during carcinogenesis. This results in defective electrical coupling between cells, which is mediated via ions and small molecules through gap junctions, which in turn influences the electrical properties of epithelia.
Epithelial cells are bound together by tight junctions, which consist of cell-to-cell adhesion molecules. These adhesion proteins regulate the paracellular transport of molecules and ions between cells and are dynamic structures that can tighten the epithelium, preventing the movement of substances, or loosen allowing substances to pass between cells. Tight junctions consist of integral membrane proteins, claudins, occludins and JAMs (junctional adhesion molecules). Tight junctions will open and close in response to intra and extracellular stimuli.
A number of substances will open or close tight junctions. The proinflammatory agent TGF-alpha, cytokines, IGF and VEGF opens tight junctions. Zonula occludens toxin, nitric oxide donors, and phorbol esters also reversibly open tight junctions. Other substances close tight junctions including calcium, H2 antagonists and retinoids. Various hormones such as prolactin and glucocorticoids will also regulate the tight junctions. Other substances added as drug formulations act as non-specific tight junction modulators including chitosan and wheat germ agglutinin.
The above referenced substances and others may act directly or indirectly on the tight junction proteins, which are altered during carcinogenesis. For example claudin-7 is lost in breast ductal epithelium during the development of breast cancer. The response of the tight junctions varies according to the malignant state of the epithelium and their constituent proteins. As a result the opening or closing of tight junctions is affected by the malignant state of the epithelium.
Polyps or overtly malignant lesions may develop in a background of disordered proliferation and altered transepithelial ion transport. Experimental animal studies of large bowel cancer have demonstrated that transepithelial depolarization is an early feature of the pre-malignant state. In nasal polyp studies, the lesions had a higher transepithelial potential, but these lesions were not pre-malignant in the same sense as an adenomatous or pre-malignant colonic polyp, that are usually depolarized. Electrical depolarization has been found in biopsies of malignant breast tissue. Recently alterations in impedance have been found to be associated with the pre-malignant or cancerous state in breast and bowel.
It has been discovered that transepithelial depolarization was a specific event associated with colonic carcinogenesis in CF1 mice. The more susceptible site, the distal colon, underwent about a 30% decrease in transepithelial potential (VT) after only four weeks of carcinogen treatment. This was before histological changes developed. A non-specific cytotoxic agent (5-fluorouracil), administered over the same period did not cause a reduction in VT in the same model. The reduction in VT was confirmed in a subsequent study where almost a 60% reduction was observed after carcinogen treatment. It has also been discovered that, although VT is invariably higher when measured in vivo, the “premalignant” colonic epithelium is usually depolarized when compared to normal colon.
DC electrical potential alterations have been used to diagnose non-malignant conditions such as cystic fibrosis, cancer in animal models, human cells or tissue and in man. Differences in impedance between normal tissue and cancer have been described in animal models in vitro human tissue in vitro and have been applied to in vivo cancer diagnosis.
DC potential measurements have not been combined with impedance measurements to diagnose cancer because the electrophysiological alterations that accompany the development of cancer have not been well understood or fully characterized. Surface measurements of potential or impedance are not the same as measurements performed across the breast epithelium, and described below, where electrical contact is made between the luminal surface of the duct and the overlying skin. Transepithelial depolarization is an early event during carcinogenesis, which may affect a significant region of the epithelium (a “field defect”). This depolarization is accompanied by functional changes in the epithelium including ion transport and impedance alterations. Early on in the process these take the form of increased impedance because of decreased specific electrogenic ion transport processes. As the tumor begins to develop in the pre-malignant epithelium, structural changes occur in the transformed cells such as a break down in tight junctions and nuclear atypia. The structural changes result in a marked reduction in the impedance of the tumor. The pattern and gradient of electrical changes in the epithelium permit the diagnosis of cancer from a combination of DC electrical and impedance measurements.
Another reason that DC electropotential and impedance measurements have not been successfully applied to cancer diagnosis is that transepithelial potential and impedance may be quite variable and are affected by the hydration state, dietary salt intake, diurnal or cyclical variation in hormonal level or non-specific inflammatory changes and other factors. In the absence of knowledge about the physiological variables which influence transepithelial potential and impedance these kind of measurement may not be completely reliable to diagnose pre-malignancy or cancer.
Furthermore, a detailed understanding of the functional and morphological alterations that occur during carcinogenesis permits appropriate electrical probing for a specifically identified ion transport change that is altered during cancer development. For example knowledge that electrogenic sodium absorption is altered during cancer development in breast epithelium permits the use of sodium channel blockers (amiloride) or varying sodium concentration in the ECM (electroconductive medium) to examine whether there is an inhibitable component of sodium conductance. By varying the depth of the measurement (by measuring the voltage drop across differently space electrodes), it is possible to obtain topographic and depth information about the cancerous changes in the epithelium. Using a combination of low and high frequency sine waves probing at different depths we are able to correlate the functional and morphological (structural) changes at different depths, with the impedance profile of the tissue.
The diagnostic accuracy of current technology using DC electropotentials or impedance alone have significant limitations. Sensitivity and specificity for DC electrical measurements in the breast have been reported as 90% and 55% respectively and 93% and 65% for impedance measurements. This would result in an overall diagnostic accuracy of between 72-79%, which is probably too low to result in widespread adoption. The measurement of ductal transepithelial DC potential, ductal transepithelial AC impedance spectroscopy alone, or the combination of DC electrical potentials and impedance spectroscopy will result in a diagnostic accuracy of greater than 90%, which will lead to improved clinical utility.
Breast cancer is thought to originate from epithelial cells in the terminal ductal lobular units (TDLUs) of mammary tissue. These cells proliferate and have a functional role in the absorption and secretion of various substances when quiescent and may produce milk when lactating. Functional alterations in breast epithelium have largely been ignored as a possible approach to breast cancer diagnosis. Breast epithelium is responsible for milk formation during lactation. Every month pre-menopausal breast epithelium undergoes a “rehearsal” for pregnancy with involution following menstruation. The flattened epithelium becomes more columnar as the epithelium enters the luteal phase from the follicular phase. In addition duct branching and the number of acini reach a maximum during the latter half of the luteal phase. Just before menstruation apoptosis of the epithelium occurs and the process starts over again unless the woman becomes pregnant.
Early pregnancy and lactation may be protective against breast cancer because they result in a more differentiated breast epithelium which is less susceptible to carcinogenic influences whether estrogen or other environmental factors. It therefore seems that differentiated breast epithelium is less likely to undergo malignant change. Differentiated epithelium has a distinct apical and basolateral membrane domain to enable it to maintain vectorial transport function (the production of milk). In addition, differentiated cells maintain a higher cell membrane potential to transport various ions, lactulose and other substances in and out of the duct lumen. In contrast, more proliferative epithelial cells have depolarized cell membranes and are less able to maintain vectorial ion transport. Recently the epithelial Na+ channel (ENaC) and the cystic fibrosis transmembrane conductance regulator (CFTR) have been identified in mammary epithelium and both localized on the apical, or luminal side, of the epithelium. These two transporters can be probed for by using amiloride, a blocker of the ENaC, or by opening up Cl− channels regulated by CFTR using cAMP.
For example, 20 μM luminal amiloride depolarized the transepithelial potential from −5.9±0.5 mV (mean±SEM) by +3.1±0.5 mV. Forskolin (10 μM), which raises cAMP and opens Cl− channels via the CFTR hyperpolarized the breast epithelium by −2.2±0.1 mV. These changes were accompanied by an increase (17%) and subsequent decrease (19%) in transepithelial resistance respectively. In transformed breast epithelium the ENaC is down-regulated, whereas Cl− secretion may increase, similar to observations reported for carcinoma of the cervix. Non-lactating breast epithelium has relatively leaky tight junctions. This results in a paracellular shunt current, which hyperpolarizes the apical membrane of the epithelial cell. The larger the shunt current the more hyperpolarized the apical membrane and therefore the epithelium depolarizes since TEP=VBL−VA and i=TEP/RS. TEP=Transepithelial potential; VBL=voltage of the basolateral membrane; VA=voltage of the apical membrane; i=shunt current; RS=paracellular (shunt) resistance.
Evidence that breast carcinogenesis may be associated with functional incompetence of breast epithelium also comes from a number of other sources. Some transgenic strains of mice have defective lactation. The transgenic src mouse which develops hyperplastic alveolar nodules, otherwise develops a normal mammary tree but has defective lactation. The notch4 and TGFβ transgenic mouse also demonstrate defective lactation. Cyclin D1 females have persistent lactation 6-9 months after weaning, and TGFα mice, which have a defect in apoptosis and fail to undergo epithelial regression develop hypersecretion. These data suggest that there is a relationship between epithelial function and genetic expression which affects proliferation and tumor development.
Breast cysts occur in 7% of the female population and are thought to develop in the TDLUs. Apocrine cysts have a higher potassium content than simple cysts. Apocrine cysts may be associated with the subsequent development of breast cancer. There may therefore be a fundamental change in the epithelium at risk for breast cancer development with a redistribution of electrolyte content across the cell membrane resulting in altered cyst electrolyte content and cell membrane depolarization. Although it is commonly known that during lactation the breast transports lactulose, proteins, fatty acids, immunoglobulins cholesterol, hormones, ions and water across the ductal and lobular epithelium and actively secretes milk, it is less widely appreciated that in the non-pregnant and non-lactating state the breast, throughout life exhibits excretory and absorptive function. The difference between the lactating and the non-lactating breast being of degree and the chemical constitution of the nipple duct fluid. Ductal secretions have been analyzed to diagnose biological conditions of the breast.
A number of approaches have been used to obtain ductal fluid, including a suction cup to obtained pooled secretions; nipple aspiration fluid (NAF), and more recently, cannulation of one of the 6-12 ducts that open onto the nipple surface. Substances and cells within the duct fluid may therefore be accessed to identify abnormalities that may be associated with the diseased state of the breast. One disadvantage of the above referenced approaches is the difficulty in obtaining adequate NAF or lavage fluid to perform analysis. Another disadvantage has been the inability to identify or cannulate the ducts where an abnormality in the fluid or cells may be identified.
Hung (U.S. Pat. No. 6,314,315) has suggested an electrical approach to identify ductal orifices on the nipple surface. In that disclosure it is taught that DC potential or impedance measurement may facilitate the identification of openings or orifices on the surface of the nipple. However, it is not taught that the characteristics of the DC electrical signal or impedance may characterize the condition of the breast. Moreover, it is not taught that breast transepithelial DC measurements, transepithelial AC impedance spectroscopy, alone or in combination may be used to diagnose breast cancer.
Ionic gradients exist between the fluid secretions within the breast ducts and the plasma. For example, it is known that the nipple aspirate fluid has a sodium concentration [Na+] of 123.6±33.8 mEq/l (mean±standard deviation) compared with a serum [Na+] of approximately 150 mEq/l (Petrakis1). Nulliparous women have NAF [Na+] that are approximately 10 mEq/l higher than parous women, but still significantly below serum levels. Similarly potassium concentration [K+] is significantly higher at 13.5±7.7 mEq/l in parous women and 12.9±6.0 mEq/l in nulliparous women compared with serum levels of [K+] of approximately 5.0 mEq/l. Other investigators have reported lower NAF [Na+] of 53.2 mEq/l suggesting that significant ionic gradients can be established between the plasma and duct lumen in non-lactating breast. In pregnancy these gradients are even higher for sodium with a [Na+] of 8.5±0.9 mEq/l reported in milk which is almost 20 fold lower than plasma. Chloride concentration [Cl−] in milk is almost one tenth of the concentration found in plasma with values of 11.9±0.5 mM reported. Although [Na+] and [Cl−] levels in ductal secretions rise and the [K+] falls following the cessation of lactation, significant ionic gradients are maintained between the duct lumen and plasma.
Furthermore, in women undergoing ovulatory cycles during lactation distinct changes have been observed in the ion and lactulose concentrations of breast milk. The first change occurs 5-6 days before ovulation and the second 6-7 days after ovulation. During these periods [Na+] and [Cl−] increased more than two-fold and [K+] decreased approximately 1.5-fold. It is unclear whether changes in estrogen or progesterone levels before and after ovulation are affecting the ion composition of milk. However, it is known that alterations in the ionic composition of milk influences the transepithelial electrical potential as measured in mammals.
Furthermore, it is known that various hormones affect breast epithelial ion transport. For example, prolactin decreases the permeability of the tight-junctions between breast epithelial cells, stimulates mucosal to serosal Na+ flux, upregulates Na+:K+:2Cl− cotransport and increases the [K+] and decreases the [Na+] in milk. Glucocorticoids control the formation of tight-junctions increasing transepithelial resistance and decreasing epithelial permeability. Administration of cortisol into breast ducts late in pregnancy has been shown to increase the [K+] and decrease [Na+] of ductal secretions. Progesterone inhibits tight-junction closure during pregnancy and may be responsible for the fluctuations in ductal fluid electrolytes observed during menstrual cycle in non-pregnant women, and discussed above. Estrogen has been observed to increase cell membrane and transepithelial potential and may stimulate the opening of K+-channels in breast epithelial cells. The hormones mentioned above vary diurnally and during menstrual cycle. It is likely that these variations influence the functional properties of breast epithelium altering the ionic concentrations within the lumen, the transepithelial potential and impedance properties, which are dependent upon the ion transport properties of epithelial cells and the transcellular and paracellular conductance pathways.
Accordingly, these variations can be used as diagnostic indicia of changes to breast tissue, which have to date yet to be exploited. Thus, there remains a need for effective and practical methods for detecting abnormal breast tissue.